Many applications of materials such as ceramics, glasses, composites, and polymers involve thermal shock conditions. Stresses may develop in the materials as a result of heating or cooling during applications. If the stresses are sufficiently high and the volume of the material under stresses is sufficiently large, the material will fracture and fail. This is called thermal shock fracture. For example, ceramic materials that are transparent in the infrared range (3-5 micrometer wavelength) are typically used as domes for guidance of missiles. During service, these materials heat up rapidly due to aerodynamic heating, and are subject to thermal shock fracture if the heating rate is too high or the material's resistance to thermal shock is too low. The present invention addresses the need of a simple thermal shock testing method for such infrared dome applications. The results of the invention, the apparatus and method are, however, not limited to the infrared dome application, and may be useful for any thermal shock environments that involve heating in which convective heat transfer is the dominant heat transfer mode.
Hasselman developed a series of equations for calculating thermal shock resistance figures of merit (R parameters) which took into account both mechanical add thermal properties of the materials: D.P.H. Hasselman, "Thermal Stress Resistance Parameters for Brittle Refractory Ceramics, A Compendium," Bulletin of the American Ceramic Society 49, 1033-1037 (1970).
These equation can be applied to a wide variety of thermal shock conditions. These theoretical figures of merit are useful in comparing the theoretical thermal shock behavior of different materials. However, they must be substantiated by the results of thermal shock testing. Therefore, simple thermal shock resistance tests for ranking materials are strongly desirable. This is especially true for the case of infrared transmitting materials for dome applications.
Traditional methods of thermal shock testings include the following: (1) quenching bars into water or oil and determining the strength reduction caused by the quenching (2) laser thermal shock testing involving rapidly heating the central portions of disk samples irradiated with high power laser, (3) wind tunnel testing of actual domes, (4) impinging cold air jet on hot disks, (5) exposing the central portions of disk samples to radiation heat flux from a high-temperature furnace, (6) rapidly heating disk samples in a lamp furnace equipped with focussing mirrors to allow very fast heating, (7) quenching bars into fluidized beds for convective heat transfer, (8) exposing samples to heating in a gas flame of a burner, and (9) fast heating using an array of solar mirrors. These testing methods were discussed in the following references: D. Lewis, "Thermal Shock Testing of Optical Ceramic," SPIE Vol. 297, 120-124 (1981); J. R. Brockenbrough, L. E. Forsythe, and R. L. Rolf, "Reliability of Brittle Materials in Thermal Shock,"
Journal of the American Ceramic Society 69[8]634-637 (1986); K. T. Faber, M. D. Huang, and A. G. Evans, "Quantitative Studies of Thermal Shock in Ceramics Based on a Novel Test Technique," Journal of the American Ceramic Society 64[5]296-301 (1981); and F. A. Strobel "Thermostructural Evaluation of Spinel Infrared Domes", SPIE Vol. 297 pp. 125-136 (1981).
The disadvantages of the above testing methods are:
(1) Quench testing--Results strongly depend on sample size, heat transfer mode complicated by water boiling, heat transfer coefficient is limited if oil is used as the quenching medium, stress corrosion effect unavoidable, and edge or corner effects are undefined.
(2) Laser thermal shock test--Large capital investment for the stable, flat-top type of laser required in the laser thermal shock test. Heat transfer mode involves radiation to the coated layer absorbing the laser radiation and conduction within the sample.
(3) Wind tunnel test--The test involves very sophisticated equipment and instruments, and the calibration and reproducibility of the test conditions are difficult.
(4) Cold-air-jet on hot disk--This test suffers from the fact that thermal conductivity of ceramics typically decreases with increasing temperature. Testing a hot disk does not stimulate the same thermal conditions in a cold disk subjected to aerodynamic heating.
(5) Furnace radiation--Usually insufficient heat flux to initiate thermal shock fracture in ceramics with reasonable strength. The heat transfer mode is primarily radiation.
(6) Lamp furnace heating--This test uses radiation heat transfer.
(7) Quenching in fluidized bed--This test utilizes a low heat transfer coefficient.
(8) Gas burner--The flame temperature is nonuniform; therefore, reproducibility is difficult. (9) Solar mirrors--The test involves very sophisticated equipment and instruments.